Statistical photoluminescence of dislocations and associated defects in heteroepitaxial GaN grown by metal organic chemical vapor deposition

Abstract

Heteroepitaxial GaN films in general have very high concentration of defects and at the same time show broad photoluminescence (PL) peaks at energies below the band-gap energy $\left(3.4\mathrm{eV}\right)$, i.e., the so-called yellow (YL) and blue luminescence (BL) centered at 2.2 and $2.9\mathrm{eV}$, respectively. The origin of these PL peaks is commonly attributed to defects. The present paper studies the relationship of the yellow and blue luminescence with the defect structure by a combination of photoetching and photoluminescence. Nominally undoped ($n$-type) GaN layers were grown by metal organic chemical vapor deposition (MOCVD). By photoetching the perfect material between defects is removed and defect-related nanocolumns are formed. PL measurements of samples etched to various degrees allow the identification of the different PL features. A fully etched sample shows no PL signature related to any of the band edge features and BL is quenched completely, while the yellow luminescence is attenuated only to the degree to which the volume amount of GaN has been removed. Such behavior suggests that defects causing YL are not related to dislocations but are rather homogenously distributed throughout the layer in the form of point-like defects. The decrease of the BL intensity at $4.4\mathrm{K}$ with excitation dose (bleaching) and the simultaneous increase in YL intensity for as-grown samples and the lack of such effects for etched samples imply a direct connection between the defect centers responsible for YL and BL.